An MRI apparatus permits imaging and spectral analysis of the internal structure of a sample to be performed in a nondestructive manner by making use of a reaction of the atomic nucleus of, for example, hydrogen atom, phosphorus atom, .sup.13 C atom and so on, with a radio frequency electromagnetic wave (RF, for example, 200 MHz for hydrogen atom) in a powerful electromagnetic field generated by, for example, a superconducting magnet, to cause resonance therewith. MRI apparatuses have been used in the medical field widely for clinical tests for the diagnosis of tumors, intracerebral bleeding and so on.
MRI permits an image diagnosis and spectral analysis of the internal structure of a sample to be inspected, based on the phenomenon of magnetic resonance (MR) of atomic nucleus of, for example, hydrogen atoms, and can realize a nondestructive inspection of the internal structure and chemical composition of the sample. It is also employed for nondestructive visual quality assessments of samples by permeative visual inspection and for analyses of the chemical composition of materials as well, since it permits permeative access through materials and provides a spectrum corresponding to the functional groups of the molecules of the material. The MRI apparatuses to be used for the nondestructive quality assessments and analyses of samples are also based on the same principle as the MRI apparatuses for medical diagnosis, so that both have nearly the same construction.
In the appended FIG. 5, a conventional MRI apparatus used for nondestructive quality assessments and analyses etc. of samples is shown in a perspective view. In FIG. 5, the numeral 1 indicates a static magnetic field generating element consisting of, for example, a superconducting magnet, which is designed in a horizontal cylindrical form and provided on its inner circumference with a gradient magnetic field generating element 2, totally supported on a supporting stand 3. In the hollow core space 4 of the static magnetic field generating element 1, a cradle 6 carrying a cylindrical probe 5 is inserted to permit its guiding into or out of the core space. The cylindrical probe 5 has internally an RF-emission-reception set including an RF coil and a condenser and, externally, a tuner shunk 7 projecting out of it. The MRI apparatus is controlled by a control device including a computer, which is not shown.
In the above MRI apparatus, a sample holder (not shown) charged with a sample is placed on the cradle 6 and the cradle thus carrying the sample is then guided into the hollow core space 4 of the static magnetic field generating element 1 and is settled in position in the gradient magnetic field generating element 2. When a static magnetic field is formed in this state by the static magnetic field generating element 1, some disturbances in the static magnetic field occur due to the presence of the sample, the cylindrical probe 5 and the cradle 6 in the hollow core space 4, so that shimming of the magnetic field to homogenize the static magnetic field has to be performed. On the other hand, the RF-emission-reception set accomodated in the cylindrical probe 5 is adjusted to the resonance frequency by means of the tuner shunk 7. Then, the gradient magnetic field generating element 2 is actuated by energizing the coil thereof while causing an RF pulse to be emitted from the RF coil of the RF-emission-reception set, in order to build up the resonance. The MR signals are received by the RF-emission-reception set while ceasing the emission of the RF pulse, whereupon the signals are processed by the computer into a series of image signals to obtain the MR image.
However, conventional MRI apparatuses for quality assessment and for analytical uses are not satisfactory in adaptability for such uses, since they are designed on the same principle as those for medical diagnosis. MRI apparatuses directed to applications for quality assessment, analysis etc. of samples which are far smaller than human bodies to be inspected in medical diagnosis should require, thus, performances different from those for medical diagnosis.
For instance, when a plurality of samples are to be inspected, the procedure of sample exchange is carried out after the cradle 6 has been withdrawn from the hollow core space. The cradle carrying now an exchanged sample is then re-inserted into the static magnetic field in the hollow core space to subject it to a further MR inspection. After the sample is reset, the exact positions of the probe 5, cradle 6, the sample and so on may slightly deviate from those in the foregoing MR inspection. The observation error caused therefrom may reach a large value for a small sample, so that the procedures of tuning and shimming must be carried out anew on each sample exchange. If the renewal of these procedures is not performed sufficiently, the accuracy of the analysis decreases and the efficient progress of tests for a large number of samples will not be attained.
The gradient magnetic field is not constantly maintained but is formed upon each observation. When a gradient magnetic field is formed in a static magnetic field, an interference occurs between the static and the gradient magnetic fields, whereby a shock wave is generated. Since, however, the formation of the gradient magnetic field is effected in response to the emission sequence of RF pulses, a problem occurs in that the shock wave will have an influence on the MR signals received after the emission of the RF pulses and a precise MR image will not be obtained due to the shock wave.